The present invention relates to an apparatus for producing thin metal sheet directly from a molten metal and, more particularly, to an apparatus of a type having a rotary drum to the surface of which a molten metal is applied through a nozzle and the molten metal is cooled and solidified on the drum surface to become a thin metal sheet.
A typical conventional apparatus for producing thin metal sheet has a rotary drum above which disposed is a crucible having heating coils wound therearound and adapted for storing molten metal. A pressure is applied to the molten metal in the crucible by a pressurizing fluid by means of a pump so that the molten metal is poured onto the surface of the rotary drum through a nozzle provided at the bottom of the crucible. The metal is then cooled and solidified on the surface of the drum to become a thin metal sheet. This type of apparatus is shown in the specification of the U.S. Pat. No. 4,077,462.
In this type of apparatus, it is necessary to prevent the molten metal from naturally flowing out of the nozzle by the force of gravity, in the early stage of operation in which the metal in the crucible is heated up to the desired temperature, or in the event where the production of the thin film is to be suspended. In such a case, it is necessary to take various measures such as stopping of the pump for supplying pressurizing fluid to the crucible while creating a vacuum at the space above the molten metal in the crucible by connecting a vacuum pump to the crucible, or to dispose a closure plate for stopping the flow of molten metal at the end of the pouring nozzle.
By these measures, however, it is not possible to completely stop the flow of the molten metal. In addition, there is a tendency that the end opening of the nozzle is clogged partially with the molten metal which is held in the nozzle end and naturally cooled during the suspension of operation. The partial clogging of the nozzle opening considerably hinders the uniform pouring of the molten metal over the entire width of the nozzle opening, resulting in a deterioration of the quality and yield. In addition, since a frequent replacement of the pouring nozzle is necessary, the efficiency of the production is considerably lowered.
In the prior art described above, the molten metal is applied in the downward direction from the pouring nozzle. In so far as the direction of application of the molten metal, there is a prior art which is constructed to direct the molten metal upward, as in the specification of U.S. Pat. No. 3,405,757. In this prior at, a molten metal charging chamber is defined by a pair of opposing rolls and a nozzle inserted close to the rolls. The molten metal, which is charged into the charging chamber, is solidified and then rolled by the pair of rolls to become a thin metal sheet. Namely, the molten metal is discharged aiming at the roll gap between two rolls which cooperate with the nozzle in defining therebetween a pouring basin. In this type of apparatus, it is necessary to work the nozzle having a complicated configuration at a high precision and to locate the nozzle accurately in the limited space between the nozzles in close proximity of the latter. In addition, the supply of the molten metal has to be done at a rate which correctly match the peripheral speed of the rolls, because the thickness of the thin metal sheet is determined by the size of the gap between two opposing rolls. If there is a shortage of the molten metal supply, the formed metal sheet will become porous. To the contrary, the molten metal supply at an excessively large rate will cause an overflow of the molten metal from the molten metal charging chamber, i.e. the pouring basin to clog the gap between the roll surfaces and the outer surface of the nozzle to interrupt the continuous production of the thin metal sheet. Further, troublesome works such as adjustment of the roll clearance, replacement with new nozzle and so forth are required for changing the thickness of the product sheet, inevitably resulting in the reduction of production efficiency.
It is, therefore, an important technical subject to be achieved to develop and realize an apparatus which will make it possible to produce thin metal sheets having high quality and large variety of thickness, e.g. about 30.mu. to 3 mm, at a high production efficiency.
In the thin metal sheet production apparatus incorporating a roll or rolls, higher production speed, i.e. a higher peripheral speed of the roll, is required as the thickness of the product sheet is reduced. In fact, the practical maximum peripheral speed of the roll is as high as 30 m/sec. Therefore, it is a key to the high precision of product sheet thickness to realize a high response characteristic of the control system for controlling the rate of the molten metal supply in response to the fluctuation of the sheet thickness.
It will be clear to those skilled in the art that a poor response characteristic results in a production of sheet to be rejected having low precision of thickness. Conventionally, it has been proposed and actually carried out to control the pressure in the crucible by a gas type pressure control valve. This valve, however, has an extremely low response characteristic and, accordingly, is not practical. Thus, it is also an important factor to be considered and achieved to obtain a sufficiently high response characteristic of the control system.
According to the above recited specification of the U.S. Pat. No. 3,405,757, the formed thin metal sheet is taken up by a take-up reel, through a guide device, after the delivery by the pair of rolls. There is a fear that a warp or deflection of the product sheet is used during the guiding, particularly when the sheet thickness is small and the speed of the sheet transfer is high. In consequence, an extraordinary effort has to be concentrated on the guide device, often incurring a difficulty in the continuous production of the thin metal sheet. Therefore, it is also an important subject to be achieved to realize a good guide and taking-up without being accompanied by the reduction of the production efficiency.
For a long-time operation for continuous production of the thin metal sheet, it is essential to continuously supply the molten metal from the crucible. In this connection, the aforementioned specification of the U.S. Pat. No. 3,405,757 discloses a molten-metal supplying device having a head tank in which a constant head of molten metal is maintained in relation to the lower end of the nozzle, the head tank being connected to the nozzle through a conduit. This device, however, has various disadvantages or shortcomings such as difficulty in control of the supply rate of molten metal at a high response characteristic due to the nature of the supply relying solely upon the head, necessity for precise control of the supply of molten metal to the tank, deterioration of quality of the product sheet due to oxidation off the molten metal because of a direct contact with the atmosphere, and so forth.
It has been proposed also to adopt a crucible having an airtight structure, in which a predetermined air or gas pressure is maintained to force out the molten metal through the nozzle onto the drum. Such a crucible, however, is still in need of a suitable device for continuously supplying the molten metal thereinto.